1. Field of the Invention
The invention relates to the field of domestic sewage treatment, and more particularly to a device and a method for sewage treatment using a constant magnetic field.
2. Description of the Related Art
Domestic sewage has good biodegradability and high contents of nitrogen and phosphorus, and biochemical methods are often used in domestic sewage treatment. Common methods for domestic sewage treatment include: oxidation ditch, sequencing batch reactor (SBR), biological contact oxidation, biological aerated filter (BAF), A/O process, and membrane bioreactor (MBR). However, as the temperature severely affects the growth, propagation, metabolism, distribution, and the size of biological populations, the efficiency of the sewage treatment plant is low in winter due to low temperature. Besides, temperature is also related to the flocculation performance of activated sludge and viscosity of the water. Thus, the sewage treatment process and parameters thereof in low temperature are significantly different from those in common temperatures.
A typical method for domestic sewage treatment in low temperature includes: a) adding a sludge collected from a bottom of an inspection hole to a biochemical incubator for aeration if the temperature is lower than 15° C.; b) adding fresh fecal effluent to form a mixture, diluting the mixture by water, slowly aerating the reactor, and replacing the water every day; c) repeating step b) so as to form a bacterial zoogloea after eight days of incubation; d) changing operation cycle of the system and the ingredients of the nutrient solution and dosages thereof, and decreasing the temperature of the sewage in the reactor; e) after ten days when the water temperature is lowered to 10° C., changing operation cycle of the system and the ingredients of the nutrient solution and dosages thereof again, and decreasing the temperature of the sewage in the reactor to realize the incubation and domestication of the bacteria populations. The method has simple and convenient operation, and materials involved therein are easily accessible. The incubation and domestication of the bacterial populations are performed in the reactor made of organic glass and plastics.
Another typical method for treatment of decentralized domestic sewage at low temperature includes: a) burying a reaction pool underground with a distance between an upper end of the reaction pool and the ground surface of 0.5 m; b) controlling an operation cycle of the reaction pool including introducing the sludge for 6 h while aerating, standing for 2 h for precipitation, decanting clarified water and discharging a resulting sludge for 2.5 h, and discharging water after stilling for 1.5 h. The invention overcomes the shortages of poor effect of the treatment on the decentralized domestic sewage of low temperature, and is suitable to treatment of decentralized domestic sewage in cold regions.
The above two methods are capable of increasing the biodegradation property of the sewage in a certain degree. However, with the improvement of the sewage treatment standard, effects of the above two methods cannot meet the standard requirements, since the biochemical reaction rate resulting from the weakened metabolism of microorganisms in low temperature cannot be completely compensated by the advantages of the processes.
A typical method for sewage treatment by using low intensity magnetic field employs SBR process. Permanent magnets are employed as a magnetic field generator. Magnetic field intensity and magnetic field type are employed as adjustable parameters (magnetic particle number is employed as the adjustable parameter when magnetic particles are added). Zeta potential, aerobic rate of the sludge, and dehydrogenase activity are employed as evaluation indicators of the stability and activity of the sludge colloid. In the presence of a certain magnetic field intensity, the COD removal rate, ammonia nitrogen removal rate, and nitrate nitrogen removal rate are increased. Particularly, when the magnetic field intensity is between 90 and 120 mT, the COD removal rate is significantly increased; when the magnetic field intensity is between 30 and 60 mT, the removal of the nitrate nitrogen is facilitated. The magnetic field has an obvious influence on the aerobic denitrification process; when the magnetic field intensity within the range of between 0 and 150 mT, the nitrate nitrogen removal rate is increased with the increase of the magnetic field intensity. However, the method is not applied in the sewage treatment in low temperature; and meanwhile, charged particles easily accumulate in one side of the magnetic field thereby affecting the mass transfer efficiency. Besides, the method doesn't realize the integration of the magnetic field and the reactor and has no magnetic shielding.